The present invention relates broadly to a control system and method for regulating the power output of the magnetron of a microwave oven in response to the state of the food item within the oven. More specifically, the present invention relates to a control system and method for regulating the defrost mode of the microwave oven wherein the microwave oven is first energized in a highpower defrost mode and then switched to a low-power defrost mode as the food item within the oven begins to thaw.
As is well known, a food item that is defrosting will have some portions that will tend to thaw and warm up more quickly than others. Under continuous application of high-power in a thaw or defrost mode these portions of the food item will thaw and become lossy causing them to absorb more microwave power and become still warmer while other portions of the food item remain in a frozen state. This phenomena may lead to a runaway condition where a portion of the food item has thawed and is cooking while other portions are still frozen and absorbing little microwave energy. The food item may become overcooked in some portions and eventually remain undercooked in other portions thereof. Such circumstances are unacceptable in today's microwave oven industry.
This problem has been dealt with in the prior art by providing for a timed defrost mode where the magnetron of the microwave oven is operated under very low average power to permit thermal diffusion to correct for spatially uneven microwave heating within the food item. Although this prior art method deals with the problems of potential runaway, as previously described, it results in extended defrost modes of operation. It is desirable, however, to minimize the defrost time.
It is also known that the reflection, or conversely the absorption, characteristics of a food item undergoes significant change as the food item passes from the frozen to the thawed state. It is also known that this characteristic of the defrosting or thawing food item can be monitored by detection of the microwave input reflection coefficient within the microwave oven. In accordance with the present invention, therefore, the microwave oven is initially operated in a high-power defrost mode. The reflection coefficient within the microwave oven is continuously monitored and compared to a value of reflection coefficient that indicates beginning of thaw or defrost in the food item. The control system of the present invention then switches the magnetron to a low-power operation to complete the thawing operation. The present invention thereby senses that part of the food item that has thawed and terminates the high-power defrost mode. The overall effect of the control system and method of the present invention is to shorten the required defrost time particularly with larger food items. Other characteristics of microwave oven operation can be determined utilizing the control system of the present invention, for example, determination of stoppage of the microwave energy mode stirrer and the values of the forward and reflected power within the microwave oven's microwave feed system.